In oilfield applications, as in many other fields, it is often important to be able to analyze the composition and properties of a multiphase fluid stream, such as a gas-condensate stream or a wet-gas stream, flowing in a pipeline. In particular it is generally desirable to know the flow ratios of the various phases in the multiphase fluid stream, such as the gas-oil ratio (“GOR”) or the condensate-gas ratio (“CGR”), so that the composition of the multiphase fluid stream may be understood for production purposes. It is also desirable to know the properties of the various phases, for example their pressure, volume and temperature (“PVT”) relationships to understand the dynamics of the multiphase fluid stream. For example, the prediction of fluid phase behavior and reservoir simulation models based on equations-of-state (EOS) typically requires high quality PVT data. PVT data is also often needed for flow assurance in wells and transport lines. Furthermore, PVT data may often have a significant impact on processing facility designs and specifications, and therefore on the profitability of gas-oil field.
However, to accurately determine the PVT properties and composition of reservoir fluid, representative fluid samples are required. Common analysis sampling procedures suffer from deficiencies in either or both of their accuracy and their ability to cope with high flow rates.
For example, a high-rate gas condensate well producing 5-100 MMscf (Millions standard cubic feet)/day with a gas-oil ratio of 3 k-100 k scf/bbl (CGR from 0 to 200 bbl/MMscf) (bbl=barrel of liquid; 1 m3=6.29 bbl), can often overwhelm a test-separator, causing liquid carry-over in the separator gas outlet line, thereby providing poor measurement of the GOR and non-representative PVT samples (if the carry-over is not measured and corrected for). This normally also results in poor recombination ratios.
Gas condensate wells are particularly problematic for sampling because large volumes of gas are associated with only small volumes of liquid and, as such, the phase behavior will be highly sensitive to the quantity and composition of the liquid phase.
Wellhead sampling is therefore considered to be the only practical method of obtaining reliable test data for such applications. Isokinetic sampling at a wellhead of a multiphase fluid is desirable since, if achieved, it means that the sample of the fluid is at the same pressure, temperature and velocity as the main flow stream, and therefore will have identical properties to the main flow stream and, as a result, measurements performed on the isokinetically attained samples are highly indicative of the flow properties of the main flow stream.
Methods and apparatus for isokinetic sampling are described for example in the co-owned published international patent application WO 2005/031311 A1. The present invention seeks to improve the methods and systems described in WO 2005/031311 A1.